Aluminum coated steel having chromium in diffusion layer



United States Patent ALUMINUM COATED STEEL HAVING CHRO- MIUM INDIFFUSION LAYER Robert F. Thomson, Grosse Pointe Woods, Mich., as-

signor to General Motors Corporation, Detroit, Mich, a corporation ofDelaware No Drawing. Application December 29, 1954 Serial No. 478,481

4 Claims. (Cl. 29-1962) This invention relates to aluminum coatedferrous metal parts and particularly to such parts having high ductilitydue to the presence of chromium in the diffusion layer between thealuminum and ferrous metal.

During the past few years, the oxidation resistance of ferrous basemetal articles has been increased appreciably by coating these articleswith a thin layer of aluminum or aluminum alloy. A thin layer of analloy of aluminum and the ferrous base metal is preferably provided ator adjacent the surface of the coated article to provide optimumoxidation resistance, particularly at elevated temperatures. UnitedStates Patent No. 2,569,097, Grange et al., discloses and claims aprocess for providing such a coating on ferrous metal articles.

Although the use of an aluminum coating on ferrous metal has proved tobe highly satisfactory for increasing oxidation resistance, theinterfacial layer of complex ironaluminum which is formed next to thesteel or other ferrous base metal is relatively brittle. Since thislayer may have a tendency to spall or flake off the base metal in someapplications, it is desirable to increase the ductility of aluminumcoated articles by reducing the brittleness of the interfacial layer.

Accordingly, a principal object of the present invention is to providean aluminum coated ferrous metal article having improved ductility atboth elevated temperatures and at room temperature. A further object ofthis invention is to provide an inexpensive process for forming anarticle which is both oxidation resistant and sufficiently ductile topermit its use in applications for which aluminum coated ferrous metalswere inappropriate heretofore.

These and other objects are attained in accordance with the presentinvention by introducing chromium into the diffusion layer of analuminum coated ferrous metal article, steel being preferably meployedas the base metal. Superior results are obtained if the aluminum coatingis applied by a hot dipping operation. The subsequent diffusion of thealuminum coating results in a thin, ductile aluminum-iron-chromium layeron the surface of the steel or other ferrous metal article.

Other objects and advantages of this invention will more fully appearfrom the following detailed description of a preferred embodiment of theinvention.

The surfaces of the articles to be coated are preferably cleaned priorto the coating and alloying operations. One satisfactory method is toclean the articles in a molten electrolytic caustic salt (such as thecommercially available product known as Kolene) at a temperature ofabout 900 F. The articles may then be washed in Water and thereafterpreferably further cleaned by acid pickling. A suitable acid picklingbath is an aqueous solution containing about 2% hydrofluoric acid, 7%sulfuric acid and 10% nitric acid. In some instances the articles mayrequire only a simple degreasing treatment in a chlorinated solventbefore being coated. Mechanical cleaning methods, such as grit blasting,sand blasting, hydroblasting,

2,917,819 Patented Dec. 22, 1959 2 etc., may be employed in some casesto supplement the chemical treatment. V A x The steps of degreasing andpickling are not essential to the process, however, and frequently thearticles to be coated are not contaminated to a sufficient extent towarrant this treatment.

After the articles have been cleaned, the portions there-' of which arenot to be coated may be treated with a suitable stop-off coating toprevent the coating metals from bonding to or alloying with the basemetal at such sur= faces. A suitable stop-off material for this purposeis a sodium silicate soluitomsuch as an aqueous solution con= taining20% to 50% sodium silicate. v

The chromium coating may be applied to the ferrous metal article by anyapropriate method. Electrodeposition is normally most convenient andleast expensive. Moreover, inasmuch as it is desirable to apply a verythin layer of chromium to the base metal, electrodeposition is mostsatisfactory because it permits precise control of the coatingthickness. A chromium coating between approximately 0.00025 inch and0.001 inch has proved to be satisfactory.

After the chromium coating has been formed on the ferrous metal, theouter layer of aluminum or aluminum base alloy is provided on thechromium coating. When an alloy of aluminum is used, it should containor more of aluminum. Hence, the word aluminum, when hereinafter used inthe specification and claims to refer to the outer coating material, isintended to include not only pure aluminum or commercially purealuminum, but also aluminum base alloys containing at leastapproximately 80% aluminum.

Alloying of the surface portion of the ferrous metal with the chromiumlayer may be carried out in any desired manner. The preferred method isto apply molten aluminum to the chrominum plated ferrous metal articleunder conditions such that the chromium will partially diffuse into theiron or steel and the aluminum will partially diffuse into the chromium.As hereinafter explained, the resultant intermeditae alloy layer shouldbe relatively thin.

Best results are obtained when the aluminum is applied by any of theprocedures described in the aforementioned Patent No. 2,569,097, Grangeet al. An especially advantageous method comprises preheating thechromium plated ferrous metal article to a temperature betweenapproximately 1280 F. and 1400 F. in a fused salt bath consistingessentially of 37% to 57% KCl, 25% to 45% NaCl, 8% to 20% Na AlF and0.5% to 12% AIF The preheated article is thereafter immersed for a shorttime in a molten bath of aluminum or aluminum base alloy at atemperature of about 1250 F. to 1325 F. Subsequently the aluminum coatedarticle is removed from the aluminum bath and rinsed for a short periodof time not in excess of approximately 15 seconds in the fluxing salt.The excess coating material is then removed by means of an air blast orby rapidly vibrating or rotating the coated article during orimmediately after removal from the aluminum bath. If vibration orrotation of the coated article is resorted to, it is preferable toperform this step while the article is immersed in the molten salt bath.7

After the excess aluminum coating metal has been removed, the coatedferrous base article is cooled or permitted to cool. The excess flux maybe removed by washing, for example, of the coated article. Water orother quenching media may be employed to cool the coated part. Generallyit is preferable to air cool the part below 800 F. followed by a waterquench.

A relatively short period of immersion in the aluminum bath is all thatis necessary or desirable. Ordinarily, the article being coated need beretained in the molten, alu- 'minum or aluminum base alloy not more thanapproximately 10 seconds, an immersion period between 5 and secondsbeing preferred at present.

insomc instances, particularly where small-ferrous base parts are to becoated in accordance with this invention, the aluminum may be applied bythe procedure described in ctr-pending patent application SN. 459,093,tiled September 29, 1954, in the names of Robert P. Thomson, Albert A.Shoudy, Jr.,-and Dean K. Hanink. This procedure involves applying thealuminum in the form of a paint or paste containing aluminum powder orapplying aluminum by a hot spraying operation. The latter practice iscommonly referred to as metallizing. ln'eitl or event the aluminumcoating is thereafter melted and diffused into the'chromium by thesubsequent plication of heat, preferably by'immersion in the molten saltflux hereinbefore' described.

As thus treated, the ferrous metal article is provided with an extremelythin and uniform layer of aluminum bonded to the ferrous base metal byan intermediate thin and uniform layer of chromium which has partiallyalloyed with both the aluminum and ferrous base metal.

The aluminum and alloy layers should be relatively thin in all cases. Ingeneral, the aluminum-iron-chromium layer should have a thickness fromapproximately 0.0005 inch to 0.002 inch, the outer aluminum layer havinga thickness not in excess of about 0.004 inch. It is preferred to havethe outer aluminum layer between 0.0005 inch and 0.0015 inch thick,however, resulting in a' combined thickness of thealuminum-iron-chromium layer and the'aluminum overlay of approximately0.001 and 0.0035 inch. A coating layer of this thickness possessessuperior yielding characteristics when subjected to repeated contactwith other materials and also has little tendency to crack, check orspall.

in some instances it may be desirable to electrodeposit a thin layer ofnickel on the surface of the ferrous article before the chromium platingoperation in order to improve oxidation resistance at elevatedtemperatures. For most applications, however, plating with nickel isunnecessary. if a nickel layer is employed between the chromium and theferrous metal, its thickness should not exceed a few thousandths of aninch.

Bend tests were conducted on panels treated in the above-describedmanner, and the results indicated that these panels possess considerableductility.

While this invention has been described by means of certain specificexamples, it will be understood that the scope of the invention is notto be limited thereby except as defined in the following claims.

I claim:

1. A method of providing a ductile, oxidation-resistant coating on aferrous metal article, said method comprising electroplating a layer ofchromium having a thicknessof approximately 0.00025 inch to 0.001 inchon a ferrous metal article, thereafter immersing said chromium platedferrous metal article in a fused salt bath heated to a temperature ofabout 1280 F. to1400 F.

and activatedby aluminum in contact therewith to-flux saidaa'rtiole,retaining said plated article in said salt bath until the surfacetemperature of said article substantially reaches the temperature ofsaid bath to partially diffuse the chromium into-the ferrous metal andto dissolve oxides of chromium in the salt, subsequently immersing saidplated article in a molten coating metal selected fromthe classconsisting of aluminum and aluminum base alloys-heated to a temperatureof about 1250 F.

'to'l1325 vF., retaining said article in said molten coating :metal fora period of time not exceeding ten seconds to thereby form on saidarticle a ductile aluminum-ironchromium interlayer having a thickness ofapproximately 0.0005 inch to 0.002 inch and an aluminum overlay having athickness between about 0.0005 inch and 0.004 inch, and removing thecoated article from said molten coating metal.

2. A method of providing an oxidation-resistant coating on a ferrousbase metal article while retaining the ductility thereof, said methodcomprising electroplating a layer of chromium having a thickness ofapproximately 0.0005 inch to 0.0001 inch on a ferrous base metalarticle, thereafter immersing said chromium plated article in a fusedsalt bath comprising, by Weight, approximately 37% to 57% KCl, 25% to45% NaCl, 8% to 20% NagAlFs and 0.5% to 12% A11 said fused salt bathbeing heated to a temperature of about 1280 F. to 1400 F.-and activatedby aluminum in contact therewith, retaining said plated article in saidsalt bath until the surface temperature of said article substantiallyreaches the temperature of said bath to partially diffuse the chromiuminto the ferrous metal and to dissolve Oxides of chromium in the salt,subsequently immersing said plated article in molten aluminum heated toa temperature of about 1250 F. to 1325 F., retaining said article insaid molten-aluminum for five to ten seconds to thereby form on saidarticle a continuous intermediate layer of ductilealuminum-iron-chromium having a thickness of approximately 0.0005 inchto 0.002 inch and an outer aluminum layer having a thickness of about0.0005 inch to 0.0015 inch, and thereafter removing the resultantaluminum coated article from said molten aluminum.

3. A method of providing an oxidation-resistant coating on a ferrousbase metal article while retaining the ductility thereof, said methodcomprising electrodepositing a thin layer of nickel on the surface of aferrous base metal article, electroplating a layer of chromium having athickness'of approximately 0.00025 inch to 0.0001 inch over said nickellayer, thereafter immersing said plated article in a fused salt bathcomprising, by weight, approximately 37% to 57% KCl, 25%- to 45% NaCl,8% to 20% Na AlF and 0.5% to 12% A11 said fused salt bath being heatedto a temperature'of' about 1280 F. to 1400 F. and activated byaluminum-in contact therewith, retaining said plated article in saidsalt bath until the surface temperature of said article substantiallyreaches the temperature of said bath to partially diffuse the chromiumand nickel into the ferrous metal and to dissolve oxides of chromium inthe salt, subsequently immersing said plated article in molten coatingmetal containing at least aluminum heated to a temperature of about 1250F. to 1325 F., retaining said article in said molten coating metal for aperiod of time not exceeding ten seconds to thereby form on said articlean intermediate layer of ductile aluminumiron-chromium-nickel having athickness of approximately 0.0005 inch to 0.002 inch and an outeraluminum layer having a thickness of about 0.005 inch to 0.004-inch, andthen removing the coated article from said'molten coating metal.

4. A ferrous metal article having a protective coating characterized byhigh ductility and oxidation resistance, said protective coatingconsisting of an intermediate diffusion layer of analuminum-iron-chromium alloy having a thickness of about 0.0005 inch to0.002 inch and i an aluminum overlay having a thickness of approximately0.0005 inch to 0.0015 inch, said article having a continuous chemicalbond at the interface of the ferrous metal and thealuminum-iron-chromium alloy layer.

References Cited in the file of this patent UNITED STATES PATENTS2,569,097 Grange et a1 Sept. 25, 1951 2,752,268 Whitfield June 26,1956

FOREIGN PATENTS 698,897 Germany Nov. 19, 1940 OTHER REFERENCES I Burns-and Schuh: Protective Coatings for Metals, Reinhold Publishing Corp,1939, pp. 212.

1. A METHOD OF PROVIDING A DUCTILE, OXIDATION-RESISTANT COATING ON AFERROUS METAL ARTICLE, SAID METHOD COMPRISING ELECTROPLATING A LAYER OFCHROMIUM HAVING A THICKNESS OF APPROXIMATELY 0.00025 INCH TO 0.001 INCHON A FERROUS METAL ARTICLE, THEREAFTER IMMERSING SAID CHROMIUM PLATEDFERROUS METAL ARTICLE IN A FUSED SALT BATH HEATED TO A TEMPERATURE OFABOUT 1280* F. TO 1400* F. AND ACTIVATED BY ALUMINUM IN CONTACTTHEREWITH TO FLUX SAID ARTICLE, RETAINING SAID PLATED ARTICLE IN SALTBATH UNTIL THE SURFACE TEMPERATURE OF SAID ARTICLE SUBSTANTIALLY REACHESTHE TEMPERATURE OF SAID BATH TO PARTIALLY DIFFUSE THE CHROMIUM INTO THEFERROUS METAL AND TO DISSOLVE OXIDES OF CHROMIUM IN THE SALT,SUBSEQUENTLY IMMERSING SAID PLATED ARTICLE IN A MOLTEN COATING METALSELECTED FROM THE CLASS CONSISTING OF ALUMINUM AND ALUMINUM BASES ALLOYSHEATED TO A TEMPERATURRE OF ABOUT 1250* F. TO 1325* F., RETAINING SAIDARTICLE IN SAID MOLTEN COATING METAL FOR A PERIOD OF TIME NOT EXCEEDINGTEN SECONDS TO THEREBY FORM ON SAID ARTICLE A DUCTILEALUMINUM-IRONCHROMIUM INTERLAYER HAVING A THICKNESS OF APPROXIMATELY0.0005 INCH TO 0.002 INCH AND AN ALUMINUM OVERLAY HAVING A THICKNESSBETWEEN ABOUT 0.0005 INCH AND 0.004 INCH, AND REMOVING THE COATEDARTICLE FROM SAID MOLTEN COATING METAL.